This invention is directed to microwave apparatus for heating liquid in closed plastic bags and in particular to apparatus for thawing a liquid which has been frozen in a plastic bag.
Most hospitals freeze and store blood plasma as well as intravenous admixtures for thawing and use at a later date. The blood plasma is quick-frozen at -80.degree. C. in plastic bags which hold 145-285 c.c. of the plasma on the average. The storage is done at -30.degree. C. and when required the bag is thawed by immersing in a hot water bath kept at 37.degree. C.
This method of thawing has various disadvantages. It takes approximately 35 minutes to bring the blood plasma or other admixtures to a reasonable transfusion temperature. This is too long for many emergency situations. Because of the length of time taken, the hospitals sometimes thaw the blood plasma in advance some of which is then wasted. Hot water baths are not always sterile and since some of the plastic bags are permeable, there is also a danger of the material getting contaminated.
As early as 1974, it has been proposed that microwaves be used to thaw fresh frozen blood plasmaas illustrated in the publication by Sherman, L. A. et al.--"A new rapid method for thawing fresh frozen plasma"--Transfusion, Vol. 14, No. 6, 1974, pp. 594-597. This idea has spread to the thawing of frozen intraveneous admixtures as described in the publication by Tomecko, G. W. et al., "Stability of Cefazolin sodium admixtures in plastic bags after thawing by microwave radiation", American J. of Hospital Pharmacy, Vol. 37, 1980, pp. 211-215; and Ausman, R. K. et al. "The application of a freeze-microwave thaw technique to central admixtures services", Drug Intelligence and Clinical Pharmacy, Vol. 14, 1980, pp. 284-287.
In the above method, the plastic bag of frozen material is placed at an appropriate location in the microwave oven and heated for a fixed time. Since the microwave power of the oven, the size and shape of the bag, and the storage temperature may vary, heating for a fixed time in a microwave oven results in an unacceptably high spread in the final temperatures of the bags. Even more serious is the problem of non-uniformity of heating of the bag. The edges, the corners and the ports tend to overheat. In most cases, the blood plasma or admixtures boils in some parts before it reaches a desireable temperature in other parts. This is highly unacceptable since the effectiveness of the plasma or admixture can be completely destroyed at these locations.